WASHINGTON — President Reagan's decision to impose a trade embargo on Nicaragua won only mixed reviews in Congress on Wednesday, but there were signs that a new majority was forming in support of the Administration's basic policy of pressure on the leftist Sandinista regime.

Since Reagan lost his request for renewed covert aid to Nicaraguan rebels in a bruising congressional battle a week ago, the mood on Capitol Hill has swung visibly against the Sandinistas and in favor of overt sanctions like the trade embargo.

Several Democrats have said they may switch from opposing funding for the rebels, known as contras , to supporting a limited program of aid, leading Administration officials to predict that they could win a new vote on the issue later this month.

Sen. Jim Sasser (D-Tenn.) and Rep. Bill Richardson (D-N.M.) also have said that they may switch, and an Administration official said that more than a dozen members of the House have indicated privately that now they are undecided. Administration-backed measures that would have given the contras $14 million in non-military aid passed the Senate and failed by only two votes in the House last week.

"My personal opinion is that we are getting there," Langhorne A. Motley, assistant secretary of state for inter-American affairs, said in response to a question at a State Department briefing about a possible shift in Congress. "It's a continuous process."

Motley, who announced Tuesday that he will leave the Administration, and other spokesmen took pains to point out that many Democrats had said they would back a trade embargo. "By supporting sanctions, a lot of these guys are signing on to the basic thrust of our policy, which is that you can't move the Sandinistas without exerting pressure on them," said a senior State Department official who refused to be quoted by name.

'Time to Get Tough'

Sens. Sam Nunn of Georgia and Lloyd Bentsen of Texas, conservative Democrats who have supported funding for the contras, applauded the economic sanctions and called for more such pressure, as did Senate Majority Leader Bob Dole (R-Kan.). "I think it's time to get tough and stay tough," Dole said.

Biden said he considers the embargo merely a gesture. "I don't think it means much," he said. "I don't think it hurts; I don't think it helps."

But several Senate liberals decried the move. "An economic embargo now, coming on top of persistent Administration efforts to overthrow the government of Nicaragua, brings us closer to active military involvement," Sen. Alan Cranston (D-Calif.) warned. "The course President Reagan is pursuing will lead inevitably to a call for U.S. military action."

"We are slamming the door on the possibility of peace," said Sen. Mark O. Hatfield (R-Ore.), chairman of the Senate Appropriations Committee and a longtime opponent of Reagan's Central American policies. "We're driving (Nicaragua) into the permanent embrace of the Russian bear."

Other liberals who said during last week's debate that they would support an embargo in preference to funding the contras remained silent on the issue Wednesday. Aides to Sen. Christopher J. Dodd (D-Conn.) and Rep. Michael D. Barnes (D-Md.) said they had no comment on the President's action.

Concerned Over Soviets

Biden and other undecided members of Congress have said they are concerned about the Sandinistas' alliance with the Soviet Union, highlighted this week by Nicaraguan President Daniel Ortega's visit to Moscow. They have also reported concern over reports that the Sandinistas are attempting to ship arms across their northern border to leftist insurgents in neighboring Honduras and by the Managua government's failure to make significant conciliatory gestures toward the United States after Congress rejected Reagan's request for aid to the contras.

White House spokesman Larry Speakes, announcing the sanctions in Bonn, where Reagan is attending an economic summit meeting, said the President intends to ask Congress again for aid to the contras. Measures to renew the aid are already in preparation in both the Senate and House.

Several members of Congress, including Nunn and Sen. Dave Durenberger (R-Minn.), chairman of the Senate Intelligence Committee, said they want to see more overt pressure against Nicaragua to shift the Administration's policy away from its earlier reliance on the officially covert pressure of the CIA-backed contras.

Nunn said he believes the trade sanctions can succeed only if they are also adopted by other countries. Although the United States has been Nicaragua's most important trading partner, only about 16% of its total trade was with the United States.

But State Department officials said there appears to be little chance of other countries joining the effort.